Since a polyethylene resin is excellent in moldability and various physical properties and has high economic efficiency and suitability for environmental issue, the polyethylene resin is used as an important material in an extremely wide range of technical fields and has been utilized in wide use applications. One field among these use applications is the field of pipe. Based on actual results concerning durability in earthquakes, use of the resin is spreading to a gas pipe, a water distribution pipe, and the like.
At present, the resin to be used as a gas pipe, a water distribution pipe, or the like should satisfy excellent long-term durability such as PE80 (MRS: Minimum Required Strength=8 MPa), PE100 (MRS=10 MPa), or preferably PE125 (MRS=12.5 MPa) as defined in ISO 9080 and ISO 12162. Further, it is necessary to have a very high hot internal pressure pipe creep performance as described in ISO 1167. In order to exhibit such a performance, since there is a tendency that durability becomes poor when fluidity is increased, a polyethylene having a low fluidity or melt flow rate (hereinafter, also referred to as “MFR”) has been unavoidably used even at the expense of productivity.
Currently, for example, in the standards of a polyethylene pipe for water distribution, in order to secure the durability, the following is defined in pipe thickness design: SDR (Standard dimension ratio)=D/t=11. However, in this case, when a pipe whose nominal diameter is larger, the pipe thickness also increases in proportion and, for example, in the case of a pipe whose nominal diameter is 200, the thickness reaches 20 mm. Therefore, for the reasons of weight increase, economical efficiency, and the like, in order to decrease the pipe thickness, it is necessary to further improve MRS that is a long-term hydrostatic strength of a polyethylene pipe material for water distribution.
In terms of pressure resistance, currently, a ductile cast iron pipe or the like having excellent pressure resistance, weather resistance, and the like as compared to polyethylene pipes have been frequently used as a water distribution pipe that can be used also under a high water pressure. Therefore, in order to use a polyethylene pipe as a substitute for the ductile cast iron pipe, in the polyethylene pipe, it is necessary to improve the pressure resistance, the weather resistance, and the like.
In addition, recently, due to the change in the construction method of pipe laying, there has been required a polyethylene resin which is also excellent in long-term durability even when the surface of a molded pipe is scratched, i.e., is also excellent in slow crack growth (Slow Crack Growth: SCG), such as a notched pipe test defined in ISO 13479.
These polyethylene resins for pipes have been produced by co-polymerization of ethylene and an α-olefin through multistage polymerization in the presence of a Phillips catalyst or a Ziegler catalyst, but the polyethylene resin produced by the Phillips catalyst has a drawback in long-term durability, so that a highly durable polyethylene resin for water distribution pipe, which satisfies PE100, is exclusively produced by the latter Ziegler catalyst.
There are numerous prior art technologies about the polyethylene resins for pipes, which are obtained by the copolymerization of ethylene and an α-olefin through multistage polymerization using a Ziegler catalyst, but it is difficult to produce a polyethylene resin that satisfies the specifications of PE100 and has excellent SCG, stiffness, fluidity, homogeneity, and the like. However, various proposals have been made as follows.
As a proposal for improving the polyethylene resins for pipes which can be produced using a Ziegler catalyst, for example, for the purpose of providing a polyethylene molding compound having a sufficiently high melt strength, which has no risk of cleavage of pipes during production or no risk of sagging problems and, at the same time, can be used in the production of a large diameter thick-walled pipe having homogeneity of the product and mechanical performance that sufficiently meets the quality standards for pipes, for example, long-term resistance to an internal pressure, high stress crack resistance, low-temperature notched impact strength, and high resistance to rapid crack growth, there has been proposed a polymer molding compound produced from 55 to 75% by mass of a first ethylene polymer (A) that is a copolymer of a 1-olefin having a total number of carbon atoms ranging from 4 to 10 and ethylene as a comonomer and has the comonomer in a ratio of 0.2 to 5% by mass relative to the mass of the first ethylene polymer (A) and a broad bimodal molecular mass distribution and 25 to 45% by mass of a second ethylene polymer (B) that is a copolymer produced from an ethylene constituting unit and a 1-olefin having a number of carbon atoms ranging from 4 to 10 and has bimodal molecular mass distribution different from that of the first ethylene polymer (A) (see Patent Document 1).
However, even the compound described in Patent Document 1 has not satisfied the requirements of higher pressure resistance and long-term durability.
Further, there have also been proposed polyethylene resins for pipes produced in the presence of a metallocene catalyst.
For example, for the purpose of providing an improved polyethylene pipe resin, there has been proposed a polyethylene pipe resin that is a polyethylene resin substantially produced by a metallocene catalyst and comprising 35 to 49% by weight of a first polyethylene fraction having high molecular weight and 51 to 65% by weight of a second polyethylene fraction having low molecular weight, wherein the first polyethylene fraction comprises a linear low density polyethylene having a density of 0.928 g/cm3 at highest and an HLMI of less than 0.6 g/10 minutes, the second polyethylene fraction comprises a high density polyethylene having a density of at least 0.969 g/cm3 and an MI2 of higher than 100 g/10 minutes, and the polyethylene resin has a density of higher than 0.951 g/cm3 and an HLMI of 1 to 100 g/10 minutes (see Patent Document 2).
However, even the polyethylene resin described in Patent Document 2 has not satisfied the requirements of the higher pressure resistance and long-term durability.
Moreover, the present applicant has previously proposed a polyethylene resin for a pipe having excellent moldability and balance between mechanical properties of stiffness and SCG and also excellent homogeneity (see Patent Document 3).
That is, in the fields of pipes, particularly water distribution pipes, for the purpose of providing a polyethylene resin that not only satisfies PE100 but also has particularly excellent SCG and sufficient fluidity, homogeneity, and the like and a method of producing the same, as well as a pipe and a joint using the resin, there has been proposed a polyethylene resin for a pipe wherein (a) a high load melt flow rate (HLMFR, HLa) is 5 to 20 g/10 minutes, (b) a density (Da) is 0.945 to 0.965 g/cm3, (c) an α-olefin content (Ca) is 0.05 to 1.5 mol %, and (d) a rupture time (T) by a notched Lander method-ESCR, HLa, and Ca satisfies a specific formula.
The polyethylene resin is preferably a polyethylene resin using a Ziegler catalyst, wherein the HLMFR, density, and α-olefin content are defined, the resin has a configuration specified by a notched Lander method-ESCR method, more preferably a specific α-olefin copolymer is combined in the resin, and the resin is produced by a particular multistage polymerization method. Particularly, PE100 is satisfied in a pipe molded article and also very excellent SCG is possible.
Patent Document 4 discloses a polyethylene for a pipe and a joint, which satisfies the following characteristics (i) to (vi).
Characteristic (i): a high load melt flow rate (HLMFR) at a temperature of 190° C. and a load of 21.6 kg is 8 to 30 g/10 minutes,
Characteristic (ii): a density is 0.947 to 0.960 g/cm3,
Characteristic (iii): a rupture time in a full notch creep test (measured at 80° C., 5 MPa) is 300 hours or more,
Characteristic (iv): a peak top time in isothermal crystallization at 121.5° C. measured on a differential scanning calorimeter (DSC) is 300 seconds or less, and
Characteristic (v): a flexural modulus (23° C.) is 950 MPa or more, and
Characteristic (vi): a melt tension at 190° C. is 100 mN or more.
However, because of the requirements of higher performance or productivity, in addition to pressure resistance, long-term durability, and fluidity, further improvement in impact resistance has been desired. Particularly, as for the pressure resistance and the impact resistance, thinning of the pipe thickness, weight reduction of the pipe, improvement in service life, expansion of applicable places, improvement of earthquake resistance, and the like can be expected, so that further improvement is particularly preferred.
Patent Document 5 discloses a polyethylene composition wherein it contains a base resin containing a fraction (A) of a low-molecular-weight ethylene homopolymer or copolymer and a fraction (B) of a high-molecular-weight ethylene homopolymer or copolymer, the fraction (A) has average molecular weight lower than that of the fraction (B), the base resin has a weight-average molecular weight (Mw) of 190,000 g/mol to 300,000 g/mol and a complex viscosity at 0.05 radian/second (η*0.05 radian/second) of 75 to 500 kPa·s, but further improvement in the performance has been demanded.
As described above, in the polyethylenes for pipes and joints and molded bodies thereof, there have been required those having a performance, which significantly exceeds a high level of the quality standards for pipes, in the pressure resistance and the long-term durability.